The present invention concerns a magnetooptic layer made from an amorphous rare-earth/transition metal alloy having magnetic anisotropy, the easily magnetizable axis of which is perpendicular to the surface.
Amorphous magnetooptic materials having such a uniaxial perpendicular anisotropy are known. The most widespread are alloys of rare-earth metals, such as gadolinium, terbium and dysprosium, with transition metals, such as iron and cobalt, to which yet further components may be added. The magnetic properties of these alloys depend very strongly on their composition.
German Offenlegungsschrift 3,309,483 describes magnetooptic recording materials made from amorphous ternary alloys based on terbium, iron and cobalt. When the cobalt components are equal to or less than forty percent of the alloy there is an approximately linear relationship of both the angle of the Kerr rotation and the Curie temperature relative to the cobalt content of the alloys. The same holds for the magnetooptic recording media described in German Offenlegungsschrift 3,536,210 and in an article in Journal of Applied Physics, 64:262 (1988). Thus, a magnetooptic recording medium made from an amorphous film composed of rare-earth/transition metals and having a compensation temperature of 50.degree. to 200.degree. C., or a compensation temperature of 0.degree. C. or less, is known from German Offenlegungsschrift 3,536,210. When an amorphous film of the Tb-Fe-Co system is used, the compensation temperature of 50.degree. to 200.degree. C. is achieved with a composition having 24 to 30 atom percent terbium, 7 to 20 atom percent cobalt, the remainder being iron, while a compensation temperature of 0.degree. C. or less is attained with a composition having 18 to 21.5 atom percent terbium, 8 to 10 atom percent cobalt and the remainder being iron. These relationships are explained in detail in German Offenlegungsschrift 3,536,210.
Starting from page 2610 of an article in Journal of Applied Physics, 61 (1987) and from page 1949 of an article in J. Vac. Sci. Technol. A5 (1987), it is pointed out that, for example, increasing the terbium content by 1 atom percent can shift the compensation temperature by up to 40.degree. C.
The control of the composition of the layer is therefore very important for design of the sputtering process and of a corresponding production plant, as discussed in Solid State Technology, March 1988, page 107.
In general, it is indicated that the deviation of the terbium concentration from the mean concentration in the layer volume is to amount to less than 0.5%.
The uniformity sought in the composition of the alloy components in the depth profile of a magnetooptic recording layer, together with the attempts to hold the alloy composition constant over the width and length of the coating require a high degree of effort, e.g., the disks to be coated rotate during the coating process about their own axis of rotation, and at the same time travel around on a sizeable circuit.
A further disadvantage of known magnetooptic recording materials is their high corrodibility.
To avoid or prevent this disadvantage, the addition of various anticorrosive substances or elements, respecitvely, to the magnetooptic allows is recommended (GB-A-2,175,160; EP-A1-0,229,292). The addition of such substances to the entire volume of the magnetooptic recording layer improves the corrosion resistance, but at the expense of other desired properties, such as high Kerr angle, high coercive field strength, high writing sensitivity, high signal-to-noise ratio and the like. In the magnetooptic recording medium according to EP-A1-0,229,292, further anticorrosive substances are added to a first substance in order to achieve an enrichment of the anticorrosive substances at the surface of the recording medium. In this process it is disadvantageous that the desirable magnetooptic properties can be even more strongly impaired through the addition of further elements.
Thin barrier layers made from anticorrosive substances are described in U.S. Pat. No. 4,740,430. A discrete multiple-layer structure of the magnetooptic recording medium is produced.
In order to achieve a high storage density of the magnetooptic recording materials, it is necessary to produce stable domains which are as small as possible in the magnetooptic recording layer. A precondition for this is that the product of the saturation magnetization M.sub.S and the coercive field strength H.sub.C be as large as possible (Kryder et al., SPIE Proc., Vol. 420, page 236 (1983)). For known magnetooptic recording materials, a product of the saturation magnetization and coercive field strength which is as large as possible is achieved only in a narrow temperature range around the compensation temperature T.sub.comp.
More recently, magnetooptic recording materials have been described that are suitable for the direct overwriting of information (U.S. Pat. No. 4,694,358, U.S. Pat. No. 4,649,519, EP-A2-0,225,141, EP-A2-0,227,480 and EP-A2-0,217,096). In all cases, use is made of a construction of the magnetooptic recording medium in which two separate layers having different magnetic properties are stratified one above the other.
The publications EP-A2-0,217,096 and EP-A2-0,227,480 describe magnetooptic recording media in which a thermally insulating interlayer is present in the construction between the magnetooptic recording layer and a magnetic layer which generates a polarizing field. In the remaining citations from the literature mentioned above such interlayers are recommended because otherwise there can be diffusion of alloying components into the magnetic layer. Naturally, such a diffusion of alloying components alters the properties of the magnetooptic recording medium.
Another way to increase the long-term stability of a magnetooptic storage device is proposed in the process according to German Offenlegungsschrift 3,642,161, in which, during and/or after the successive deposition of a dielectric layer, a magnetooptic layer and a cover layer on a substrate, a curing treatment is carried out in a virtually dry atmosphere in a temperature range from room temperature to just below the crystallization temperature of the magnetooptic layer.
There is known from Japanese Published Specification 188,843/88 a process for fabricating a photomagnetic disk, in which the photomagnetic recording layer is sputtered on in such a way that the substrate moves past three targets made from rare-earth metal and transition metal. The central target is arranged parallel to the transport track of the substrate, with a target being mounted in front of and behind the central target in the direction of transport of the substrate at a predetermined angle with respect to the central target. The composition of the photomagnetic recording film thus obtained on the substrate is uniform.